PDCD1 and histiocytic neoplasm: A prior study of a variety of human histiocytic neoplasms found that the mutational burden in one case of HS was higher than in other histiocytic tumors but that expression of PD-L1 in HS was the second lowest of all tumors, leading the authors to posit that HS patients may benefit from PD-1:PD-L1 blockade [13].